Cesium iodide interference filter with moisture resistant layer

ABSTRACT

An interference multilayer filter for use with infrared radiation comprising a system of alternating layers of materials having different refractive indices deposited on a substrate at least some of the layers being of cesium iodide.

United States Patent [72] Inventors John Scott Sceley 1 ChurchillCrescent, Sonning Common, Reading; Stanley Desmond Smith, 21, Simon sLane, Wokingham, Reading; Frederick Staflord Ritchie, Walkergate,Newcastle upon Tyne 6, all of England [21] Appl. No. 858,302 [22] FiledSept. 16,1969 [45] Patented Oct. 19, I97I [32] Priority Oct. 23, I968[33] Great Britain [31 50,381/68 [54] CESIUM IODIDE INTERFERENCE FILTERWITII MOISTURE RESISTANT LAYER 4 Claims, I Drawing Fig.

[52] US. Cl. 350/1, 117/333, 350/166 [51] Int. Cl G02b 5/28 [50] FieldolSearch 35011.163, 166

[56] References Cited FOREIGN PATENTS 742,530 l2/l955 Great Britain350/166 1,098,305 l/l968 GreatBritain OTHER REFERENCES Baumeister, Div.,Notes on Multilayer Opt. Filters," Inst. ofOptics, Mil-HDBK-MO, pp. 20-l5 8:. l6.

McCarthy, D. E., The Reflection and Transmission of Int'rared Materials:I, Spectra from 2- 50 Microns," Applied Optics, Vol. 2, No. 6, June1963, pp. 591- 595, Copy in Op 259.

Steudel, A., Preparation et Proprietes de Couches Reflechissantes pourle F abry-Pevot dans L' Ultra-violet," Le Journal de Physique et leRadium, Vol. 19, Mar. 1958, pp. 312 318,C0py in Op 259, Ocl,.l8.

Stotz et al., Fabry-Perot-Interferomcterverspiegelungen...," Feitschriftfur Physik, Vol. l5l, I958, pp. 233- 240, Copy in Op 259, 0C1 Z41.

Primary Examiner-David Schonberg Assistant Examiner-Toby H. KusmerAttorney-Holman & Stern ABSTRACT: An interference multilayer filter foruse with infrared radiation comprising a system of alternating layers ofmaterials having different refractive indices deposited on a substrateat least some of the layers being of cesium iodide.

C ESIUM IODIDE INTERFERENCE FILTER WITH MOISTURE RESISTANT LAYER Thisinvention relates to interference filters for use with infraredradiationlrgerferggg: filters are well known and commonly used devicesfor passing radiation of selected wavelengths andsimultaneouslyrefiecting and/or absorbing unwanted wavelengths. Suchfilters consist essentially of alternating layers of a material having ahigh index of refraction usually above 2, and a material having a lowindex of refraction usually below 2. The filters are constructed byevaporating the materials in a high vacuum and depositing them on to asuitable substrate.

Although the theory of optical interference filters is well establishedand understood, there are practical difficulties in the design andmanufacture of such filters for use with infrared radiation andespecially for wavelengths greater than and up to at least 80 4.. Theseproblems are especially acute with respect to the low refractive indexmaterial. lt is essential that the absorption in the materials be lowthroughout the desired wavelength range. lt is also important that thematerials can be deposited on the substrate in the relatively largethicknesses which are required without parts of the layers breaking off.

According to the present invention an interference multilayer filter foruse with infrared radiation comprises a system of alternating layers ofmaterials having different refractive indices deposited on a substrate,at least some of the layers being of cesium iodide.

Cesium iodide which has a refractive index of approximately 1.7 at awavelength of p. is used as the low refractive index material and thehigh refractive index material, may be of germanium, silicon, telluriumor lead telluride. The refractive index of germanium is of the order of4.

The substrate may be of germanium, silicon or crystal quartz, germaniumbeing preferred for the wavelength region l.720p. and silicon for thewavelength region greater than 40p.

The use of cesium iodide minimizes absorption in the wavelength range ofoperation and it can be deposited in the required thickness withoutparts of the layers breaking off to any significant extent.

The coefficient of linear expansion of cesium iodide is significantlyhigher than the coefficient for the other materials used for layers andsubstrate. For example, the coefficient for cesium iodide is almosteight times greater than that of germanium and such difference can leadto undesirable thermal stress in the multilayer filter. In accordancewith a further feature of the invention this difiiculty may be overcomeor minimized by heating the substrate during deposition and wherenecessary varying the temperature of the substrate during deposition oflayers of difierent material.

To protect the cesium iodide layers against effects of atmospherichumidity, the surface of the filter may be coated with a protectivelayer, for example, a layer of transparent polystyrene.

ln carrying the invention into effect in one form by way of example asshown in the accompanying diagrammatic drawing, an interference filterfor use in the range 5-30p. was formed by deposition on a germaniumsubstrate 1 in vacuo, alternate layers of cesium iodide and germanium 2and 3 respectively. The first germanium layer was deposited at asubstrate temperature in the range 220-230 C. and all the cesium iodideand subsequent germanium layers were deposited at l30 C. Aftercompletion the filter was allowed to cool down slowly in vacuo to about20 C. at a rate not in excess of 0.5" C. per minute.

in another example a multilayer filter for' use in the wavelength range30l00pconsisted of alternate layers of germanium and cesium iodidedeposited on a silicon substrate. The first two germanium layers weredeposited at a substrate temperature of 220-230 C. and all the layers ofcesium iodide and the remaining germanium layers at a temperature of C.The rate of cooling after completion was kc t below 0.5" C. per minuteand the filter allowed to cool to 28 C. before air was admitted to thevacuum chamber.

The temperature cycling of the substrate in the above examples canproceed at a faster rate than the final rate of cooling. Typically,heating and cooling rates of H? C. per minute can be tolerated duringthis part of the process.

A filter for use in the wavelength region l-5u consisted of alternatelayers of germanium and cesium iodide deposited in vacuo on a substrateof germanium. All the layers were deposited at 130 C. and the substrateallowed to cool down in vacuo to ambient temperature 20 C. with a rateof cooling not exceeding l C. per minute.

In a further example, a filter for use in the range 4-80 was formedusing a silicon or germanium substrate and alternate layers of cesiumiodide and lead telluride deposited at l40 C. in vacuo. The filter wasallowed to cool to ambient temperature (20 C.) at 1 C. per minute. Forthe range 4-20 a germanium substrate was used otherwise the temperaturewas the same.

Because cesium iodide is slightly hygroscopic and very soluble in water,the filters for use in the range 20-80;; were protected by coating thesurface of the filter with polystyrene provided the polystyrene does nothave absorption bands which would affect the performance of the filter.In a typical case a 4 percent solution of polystyrene in toluene wasbrushed evenly over the filter surface the toluene quickly evaporatingto leave a thin layer of polystyrene.

The thickness of the polystyrene coating can be increased by repeatedapplication of the solution. Polystyrene is transparent for wavelengthsgreater than l6 so that it can be used as a protective coating for thefar infrared region.

While in the above examples cesium iodide has been used with layers ofgermanium and substrates of germanium, materials such as silicon orcrystal quartz can be used for the substrate and silicon, tellurium orlead telluride for the layers.

We claim:

I. An in fl' a ed interference multilayer filter comprising a system ofa temating thin layers of materials having different refractive indicesdeposited on a substrate, at least some of the layers being of cesiumiodide, said substrate being a member of the group consisting ofgermanium and silicon, the surface of said filter being coated with amoisture resistant layer, said moisture resistant layer beingtransparent polystyrene.

2. A filter according to claim I, wherein alternating layers of cesiumiodide are germanium provided on a germanium substrate.

3. A filter according to claim I, wherein alternating layers of cesiumiodide and germanium are provided on a silicon substrate.

4. A filter according to claim I, wherein alternating layers of cesiumiodide and lead telluride are provided on a silicon substrate.

2. A filter according to claim 1, wherein alternating layers of cesiumiodide are germanium provided on a germanium substrate.
 3. A filteraccording to claim 1, wherein alternating layers of cesium iodide andgermanium are provided on a silicon substrate.
 4. A filter according toclaim 1, wherein alternating layers of cesium iodide and lead tellurideare provided on a silicon substrate.